In recent years, a light-emitting element using a luminous organic compound or a luminous inorganic compound as a light-emitting substance has been actively developed. In particular, a light-emitting element called an EL element has a simple structure in which a light-emitting layer containing a light-emitting substance is provided between electrodes, and has attracted attention as a next-generation flat panel display element because of its characteristics such as a thin shape, lightweight, high response speed, and direct current driving at low voltage. Further, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Furthermore, such a light-emitting element is a plane light source, and therefore expected to be applied to a light source such as a backlight of a liquid crystal display device or a lighting apparatus.
In the case of using a luminous organic compound as a light-emitting substance, the emission mechanism of a light-emitting element is a carrier injection type. In other words, a light-emitting layer is interposed between electrodes, and a voltage is applied, whereby carriers (holes and electrons) injected from the electrodes are recombined to place the light-emitting substance in an excited state. Upon return from the excited state to the ground state, light is emitted. Further, as the types of excited states, there can be a singlet excited state (S*) and a triplet excited state (T*). Furthermore, it is considered that the ratio of S* to T* in a light-emitting element is statistically 1:3.
In general, the ground state of a luminous organic compound is a singlet excited state. Thus, luminescence from a singlet excited state (S*) is referred to as fluorescence because of electron transition in the same multiplet. On the other hand, luminescence from a triplet excited state (T*) is referred to as phosphorescence because of electron transition between different multiplets. Here, in general, not phosphorescence but only fluorescence is observed from a compound emitting fluorescence (hereinafter referred to as a fluorescent compound) at room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% on the basis that S*:T*=1:3.
On the other hand, when a compound emitting phosphorescence (hereinafter referred to as a phosphorescent compound) is used, an internal quantum efficiency of 75% to 100% can be theoretically achieved. That is, emission efficiency can be three to four times as high as that of a fluorescent compound. From these reasons, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been proposed (see Non-Patent Document 1 and Non-Patent Document 2, for example). Note that Non-Patent Document 1 employs an iridium complex, in which a ligand is 2-phenylpyridine (Ir(ppy)3), as a phosphorescent compound and that Non-Patent Document 2 employs an iridium complex, in which a ligand is 2-(2′-benzo[4,5-a]thienyl)pyridine ([btp2Ir(acac)]), as a phosphorescent compound.
Further, a light-emitting element is disclosed using a light-emitting layer in which, in order to improve the lifetime and efficiency of the light-emitting element using a phosphorescent compound, an organic low molecular hole-transporting substance and an organic low molecular electron-transporting substance are contained as a host material for a phosphorescent dopant (see Patent Document 1).    Non-Patent Document 1: Testuo TSUTSUI et al., JAPANESE JOURNAL OF APPLIED PHYSICS, vol. 38, 1999, pp. L1502-L1504    Non-Patent Document 2: Chihaya ADACHI et al., APPLIED PHYSICS LETTERS, vol. 78, 2001, No. 11, pp. 1622-1624    Patent Document 1: Japanese Translation of PCT International Application No. 2004-515895